Biology may be defined as the science on life and living organisms. It is essentially a science of observation and experiment and comprises the study of the structure, origin, development, functions, and relation to environment of plants and animals, discussing at the same time the causes of these phenomena. Biology is obviously divided into zoology (zoon, "animal") and botany (botane, "herb"), according as the organism is either an animal or a plant. The biology of man is called anthropology (anthropos, "man") which, as far as it concerns man's body, is a subdivision of zoology. The science of insects is called entomology (entomon, "insect"). Biology is not a science of yesterday, but is as old as the human race. Its main development, however, took place during the last centuries. As a result of this development a great number of daughter-sciences have sprung into existence, each commanding its own more or less distinct field of research, and all united again to approach more and more the nature of life and to give us a clearer and more comprehensive idea of the variety and causes of vital phenomena.

An organism, be it plant or animal, may be considered under a threefold aspect: either in its structure, or in its functions, or in its development. And the science of biology is divided, correspondingly.

Branches and subdivisions

The science which describes the structure of organisms is called morphology (morphe, "shape"). This may be either external or internal, and either simply descriptive or comparative. But in every case morphology concerns itself only with structure, in so far as this is a definite arrangement of matter.

External morphology treats of the size and shape of external parts and organs. Its chief purposes are, first, the identification of plants and animals according to certain systems of classification and, secondly, to facilitate the study of the functions of the various organs which it describes. It is practically the same as systematic biology, which treats of the kingdoms, classes, orders, families, genera, species, and varieties of organisms.

Internal morphology studies the interior structure of organisms and their parts; that is, organs, tissues, and cells. Accordingly it is subdivided into anatomy (anatemno, "cut up"), dealing with the gross structure of organisms, histology (istos, "web"), with the minute structure of the tissues, and cytology (kytos, "cell"), with that of the cells, which are the ultimate structural and functional units of life.

Secondly, there are two sciences which refer to the functions, or activities, of organisms, according as these are performed by the single parts of the organism or by the organism as a whole. The latter science is called bionomics; the former physiology. Both physiology and bionomics not only describe and compare, but also inquire into the proximate causes of the various activities, and are thus intimately related to physics and chemistry, and at the same time are of paramount importance for the philosophy of life and of plant and animal activity. Bionomics (sometimes called æcology) observes how an organism acts with regard to its environment; that is, it describes the mode of nutrition, dwelling-place (oikos), propagation, care of offspring, peculiar relation to certain classes of other organisms (symbiosis), geographical and geological distribution, and so forth. Physiology explains in detail how the single organs, tissues, and cells discharge their manifold functions, how a muscle contracts, how a gland pours out its secretion, and whether such functions are due to physical and chemical forces, whether and how far they are subject to a special directive.

Thirdly, the several biological sciences which describe the development of organisms are comprised under the general name of morphogeny (morphe and genea, "origin"), or biogeny. The two branches of morphogeny are ontogeny (ont-, participial stem, "being") and phylogeny (phylon, "race", "stock"). The former traces the gradual development of a single individual from the egg to the perfect being; the latter, that of the so-called "systematic species" from its ultimate ancestor, from which it is supposed to have been derived by evolution. Embryology is a special branch of ontogeny, and describes the gradual differentiation of the fertilized ovum until it has attained the structure peculiar to the particular organism.

Supplementary to the biological sciences above enumerated is the science of palæontology, which describes the fossil forms of plants and animals buried and petrified in the strata of the earth. The sciences of pathology, teratology, and numerous others, which pertain rather to medicine, cannot be considered here.

The historical development

The historical development of the biological sciences may aptly be divided into four great periods: the first centering around Aristotle, Galen, and Albertus Magnus; the second commencing with Vesalius; the third, with Linnæus; the last with the theory of the cell, established by Schwann.

First period

Aristotle (384-322 B.C.) laid the foundations upon which the magnificent edifice of biology has been constructed. His works, "De historiâ animalium", "De partibus animalium", and "De generatione animalium", contain the first scientific attempt to classify animals and to explain their various biological and physiological functions. Aristotle enumerates in his works about 500 kinds of animals. He distinguished groups (gene) from species (eide), divided all animals into animals with blood (enaima) and animals without blood (anaima), and again into eight principal groups, and thus established a system of classification which is still maintained, at least in a corresponding form, in our own days. He also knew many physiological facts, and made several discoveries in bionomics which were rediscovered only in the nineteenth century. The influence of the great Stagirite upon posterity was very great, and for nearly 2,000 years most students of biology were more or less satisfied, like the younger Pliny, to study and commentate the works of Aristotle. In morphology and physiology, however, a considerable advancement was made by Claudius Galen, who was born in A.D. 131. Galen was a Greek by birth and later on a well-known physician in Rome. He was the first to define physiology as the science which explains the functions of the single parts (usus partium) of an organism.

Together with Aristotle's works Galen's morphological and physiological teachings reigned supreme in all the schools of the Middle Ages till the time of Vesalius. Only among the princes of Scholastic philosophy were there any who stepped out of the narrow circle of Aristotelean biology and commenced to study and interpret anew the living book of nature. We refer here mainly to the Dominican, Blessed Albertus Magnus (1193-1280) and to his pupils, Thomas of Chatimpré and Vincent of Beauvais. Blessed Albertus wrote seven books on plants and twenty-six books on animals. Of the latter works, some are based on original research, while others contain many new and accurate observations which today are becoming more and more highly appreciated by scientists.

Second period

The second period began with the Belgian anatomist Andreas Vesalius, born at Brussels, 1 January  5. Vesalius was the first who dared to oppose energetically the authority of Galen in certain anatomical questions and to insist that in such matters not the method of interpretation, but that of dissection and of personal observation alone could lead to truth and progress. In 1537 Vesalius was made Doctor of Medicine in the University of Padua, where, during the following five years he conducted the public dissections. At the end of this time he published an illustrated folio on the structure of the human body, "Fabrica humani corporis", which appeared at Basle in 1543. In this famous volume Vesalius corrected many errors of Galen, introduced his new method of dissection and experiment in the study of anatomy, and thus became the founder of modern anatomy. The attempt of Vesalius to overthrow traditional methods met with much encouragement, but much more opposition, apparently, for a year after the publication of his "Fabrica" he accepted the post of court physician offered to him by Charles V. In 1563 he made a pilgrimage to Jerusalem, and on his way back, in 1564, died on the island of Zante.

One of the greatest successors of Vesalius was William Harvey, born at Folkestone, England, in 1578. Harvey studied medicine at Padua at the time when the TuscanFabricius ab Aquapendente (1537-1619) held the chair of anatomy and wrote his exposition of the Galenic doctrine concerning the circulation of the blood. In 1604 he joined the Royal College of Physicians in London. Later on he became physician to Charles I, and died 3 June, 1667. The importance of Harvey's work for biology consists in the demonstration of the true circulation of the blood through the arteries and veins. This demonstration, which he developed for the first time in his anatomical lectures at the Royal College in the year 1615, was published in 1628 under the title of "Exercitatio de cordis motu". Together with the discovery of the lymphatics by Aselli (1623), to which it gave rise, it constitutes the beginning of modern physiology whose existence and development is in no small degree due to the purely experimental method definitely introduced by Harvey.

Meanwhile Galileo Galilei had made his discoveries in physics, and it was not long before these discoveries began to exercise their influence upon biological studies. It was especially Giovanni Alphonso Borelli, born at Naples, 28 January, 1608, who successfully attacked the mechanical problems suggested by muscular movement. When professor of mathematics at the University of Pisa he became acquainted with Marcello Malpighi, of Bologna, through whom he became interested in anatomical studies, and soon set about preparing a treatise on animal motion, "De motu animalium", which was the first of the great contributions to physical physiology. This influential work appeared in 1680, shortly after the death of its author. While Borelli was still at work on his "De motu", another anatomist, Nicolaus Stenson, or Steno (1638-86) developed in the same line, together with his friend Malpighi, the special physiology of glands and tissues. Steno, a convert from Lutheranism to Catholicism, was professor of anatomy in Copenhagen, his native city, and afterwards a priest and bishop in Hanover. He was one of the first to recognize the importance of the rising science of chemistry, although his attention was too much occupied with the new science of geology, which he had founded, to leave him much time for other investigations. The introduction of chemical methods in biological studies had already been accomplished by Jan Baptista van Helmont, born at Brussels in 1577, who in his turn was greatly influenced by the fantastic pilgrim Paracelsus (Theophrastus Bombast von Hohenheim), and through him by the Benedictinemonk Basil Valentine. The latter lived about the time of Johannes Gutenberg and is known as the last alchemist and the first chemist.

Van Helmont's important work, "Ortus medicinæ" appeared four years after his death, but it was the first of its kind and, like Borelli's book, exercised an important influence on future investigations. The most valuable idea of the "Ortus medicinæ" is the explanation of digestion by fermentative processes. Perhaps the most influential of van Helmont'sintellectual descendants was Franz de la Boe, or Franciscus Sylvius, professor of medicine at Leyden from 1658 till his death in 1672. Sylvius was the teacher of such brilliant men as Steno and Regner de Graaf, to whom we owe several important biological discoveries. Without making any great discoveries himself he succeeded in directing the attention of physiologists, much more than van Helmont had done, to the importance of chemistry for the solution of biological problems. Thus he became the founder of the iatro-chemical school which, in opposition to the iatro-physical school of Borelli's followers attempted to explain all vital processes by mere chemistry.

The work of Malpighi both closes this second period in the history of biology and reaches far out into modern times. Marcello Malpighi was born at Crevalcore near Bologna, in 1628, the year in which Harvey published his essay on the circulation of the blood, He did more for the general advancement of biology than any other scientist since the days of Vesalius. With the Englishman Nehemiah Grew, he laid the foundation of vegetable morphology. His work on the silkworm argues him a remarkable anatomist, and his description of the development of the hen's egg entitles him to be considered the first embryologist. But his most important work consists in the discovery of the capillaries and the air-sacs in the lungs, and of the structure of glands and glandular organs. During the greater part of his splendid career Malpighi was professor of medicine at Bologna. In 1691 Pope Innocent XII called him to Rome to be the papal physician; Malpighi complied with the invitation, and died at Rome, 28 November, 1694. A great part of Malpighi's success was due to the fact that the microscope, one of the most important scientific instruments of modern times, had just been invented.

It is noteworthy that nearly all the great pioneers of biological progress during this second period were devoted Catholics. The Church never hampered these great scientists, so long as they proceeded by way of exact demonstration, and kept within their own province, but left them perfectly free in their investigations. The exceptional ecclesiastics who assumed an unfriendly attitude towards scientific enlightenment may well be excused when we consider, as a mere physiological fact, how deeply inherited conceptions take root in the individual mind, and, moreover, how easily any novel idea may be misinterpreted as conflicting with religious truth. But the most determined opponents of biological innovations were indeed not ecclesiastics at all, but professors of biology who found it hard to give up the ancient traditions of their lifelong study.

Third period

Of Linnæus (Karl von Linné) it has been said that he found natural science a chaos and left it a cosmos. The son of a Protestant minister, he was born 23 May, 1707, at Rashult in the south of Sweden; died 1778. In 1741 he was made professor of medicine, and a little later of botany, in the University of Upsala, of which he was an alumnus. His main work, "Systema naturæ", was published for the first time in 1735. Its most complete edition is the 17th, which appeared ten years after the author's death. As its title indicates, the work is essentially a system of classification, comprising all the minerals, plants, and animals known in Linnæus' time, arranged according to classes, genera, and species. The value of this classification is mainly due to the precision of its new nomenclature. According to this "binomial" nomenclature each plant or animal received a generic and a specific name, as, for instance, Felis catus and Felis leo, indicating at once the systematic relation of the organism. Linnæus exercised a vast influence upon the biologists of his time and considerably furthered the collection of numerous morphological facts which served the great scientists of the following century as the foundation of their various theories.

To the Frenchman, Marie-François-Xavier Bichat (1771-1802), morphology owes its position as a logically co-ordinated science. Bichat was the first to introduce into biology the distinction between systems composed of heterogeneous organs and systems composed of homogeneous tissues. In a system of the former class all the organs serve some particular group of vital functions, as, for instance, the digestive system. The latter class of systems comprises all tissues which have an identical structure, as, for instance, the system of secretion. To the scientific principle established by Bichat two others were soon added which are of still greater importance in morphology. These are the laws of correlation and of homology of organs. According to the law of correlation there is a certain interdependence of all the organs of an animal, so that from the peculiar structure of one organ we may conclude as to the structure of most other organs. The law of the homology of organs maintains that all organs constructed according to the same pattern must have similar functions. But, as the same function is not necessarily bound to the same structure (e.g., the function of breathing, which may be accomplished by gills as well as by lungs), the law was complemented by the principle of the analogy of organs.

These highly suggestive laws were chiefly established by George Dagobert Cuvier  like Linnæus, a devout Protestant  who was born in 1769 at Mömpelgardt, Würtemberg, and died, a peer of France, in 1832. His chief works were written when he was professor of comparative anatomy at the Jardin des Plantes in Paris. In Cuvier's mind originated the celebrated theory of types, which was established in the year 1812. Taking the principle for the new division of the animal kingdom from the peculiar organization of the animal, Cuvier comprises the classes of mammals, birds, and reptiles under the name of vertebrates, which had shortly before been introduced by Lamarck. The other classes of animals were divided into three provinces (embranchements), the molluscs, the articulates, and the radiates. As the doctrine of the constancy of species, Cuvier's system was opposed by Etienne-Geoffroy Saint-Hilaire (1722-1844), who emphasized the universal unity of the plan of structure pervading the animal kingdom. Cuvier also made an extensive study of the petrified organisms of prehistoric ages, and thus became the founder of the science of palæontology. Cuvier's system was further developed by C. E. von Baer (1792-1876), who discovered the mammalian ovum, and through his studies of the development of the chick laid the foundations to the science of comparative morphogeny.

During the same period of the eighteenth century the science of physiology made considerable progress though the work of Boerhaave, Stahl, and Haller. Hermann Boerhaave (1668-1738) was for a long time professor of medicine at Leyden. He was an adherent neither of the extreme chemical nor of the extreme physical school, but tried to reconcile both doctrines. His main work, "Institutiones medicæ", was published in 1708. A similar position as to the causes of physiological phenomena was assumed by George Ernest von Stahl (1660-1734), famous in the annals of chemistry for his phlogiston theory. Stahl's views were embraced by a pupil of Boerhaave, Albrecht von Haller (1708-77), who united in his voluminous work, "Elementa Physiologiæ corporis humani", all the theories and discoveries known to his time, and grouped them in a new manner, so that his book may be called the first modern textbook of physiology. About the time when Haller died Antoine-Laurent Lavoisier (who was guillotined by the Convention in 1794) added to the sum of physiological knowledge by solving the problem of oxidation and respiration.

Fourth period

Meanwhile another important discovery had been made which gradually inaugurated the fourth and most splendid period of biology, the chief activities of which centre about the structure and functions of the cell, and about individual and specific evolution. During the same period immense progress has been made in bionomics, palæontology, morphology, physiology, and, indeed, all biological sciences. The fact has already been alluded to that, towards the close of the sixteenth century, a native of Holland, Zachary Janssen, had invented the microscope, which, after it had been considerably improved by Francesco Fontana, of Naples, and Cornelis van Drebbel, of Holland, was used by Malpighi, Jan Swammerdam (1627-80) of Amsterdam, the Englishmen Hooke and Grew, and by Antonius von Leeuwenhoek (1632-1723), the famous discoverer of the infusorians. Robert Hooke (1635-1702) was the first to represent in his "Micrographia" a group of cells which he had discovered with his microscope in plants; but Malpighi and Grew are generally credited with having discovered the cell. About a century later Kaspar Friedrich Wolff published his important "Theoria generationis" (1759) which clearly shows that he must have observed cells in plants as well as in animals. All this, however, was but preliminary; the new era in biology was fairly opened only when, in the years 1838 and 1839, the botanist Schleiden and, especially, the zoologist Schwann, established the first theory of the cell: that the cell is the ultimate structural and functional unit of life. Theodor Schwann was born at Neuss, near Cologne, in 1810 and became professor of anatomy at Louvain in 1839, and at Liège in 1848, and died in 1882. He was a faithful Catholic throughout his life. Schwann's theory was further developed by F. Leydig (1857), by M. Schultze (1861), and by a host of such eminent scientists of the present generation, as J. Reinke, O. Hertwig, Waldeyer, Edmund B. Wilson, and many others. The name histology (see definitions at beginning of this article) was introduced by K. Meyer* in 1819, whilst John B. Carney, who died in 1899 as a Catholicpriest and professor at Louvain, is the acknowledged author and able promoter of cytology.

Together with cytology there came into prominence the science of ontogeny which has led many biologists of today back to a vitalistic conception of the phenomena of life. This science it was that suggested E. Häckel's biogenetic law, to which it also gave the deathblow. According to Häckel's theory, ontogeny is said to be a short and rapid repetition of phylogeny. The first to trace the entire development of all the tissues from the germ cells was Schwann. The question: whether the embryo was preformed in the egg and originated by a simple evolution; or whether it had to be developed by an entirely new formation, or epigenesis; was mainly solved by the theory of epigenetic evolution established by Driesch and numerous colabourers. The science of phylogeny began when Lamarck, the founder of the modern theory of descent, controverted the immutability of species on scientific grounds.

The Chevalier de Lamarck (Jean-Baptiste-Pierre-Antoine de Monet de Lamarck) was born in 1744. At the age of forty-nine he became professor of the zoology of invertebrates in the Jardin des Plantes at Paris. His theory of evolution was fully explained for the first time in his "Philosophie zoologique" and later in his "Histoire naturelle des animaux sans vertèbres". During the last seventeen years of his life Lamarck was blind and lived in extreme poverty. The last two volumes of his "Histoire naturelle" he dictated to an affectionate daughter, who remained at her father's side till his death in 1829. During its first period of energetic development the theory of evolution, as proposed by Lamarck and, in a modified form, by Saint-Hilaire, failed to supersede the theory of the constancy of species, which was defended by such influential men as Cuvier; nor, indeed, were the facts known at that time in any way sufficient to ensure its acceptance. However, after Charles Darwin had published his "Origin of Species", in 1859, the new science progressed with the greatest rapidity, and at the present day there are but few prominent naturalists who do not contribute their share to phylogeny. At the same time it has gone through a considerable intrinsic development, mainly with respect to the rise and decline of the theory of natural selection as the chief factor in the development of species. Charles Darwin was born at Shrewsbury in 1809. He studied at the universities of Edinburgh and Cambridge, from 1831 to 1836 accompanied an English scientific expedition on board the "Beagle", and passed the rest of his life in the village of Down, Kent, where he produced the numerous works which had such an incalculable influence on his age. Among Darwin's fellow-workers Alfred Russel Wallace (born 1822) occupies the first place, since he was the co-discoverer of the principle of natural selection. Other distinguished men who took part in the development of this branch of biology were Huxley, Lyell, Nägeli, Weismann, Asa Gray. Probably the most important discoveries were those made by Hugo De Vries and by Gregor Johann Mendel, Abbot of the Augustinian Monastery at Altbrünn, where he died in 1884. Mendel's laws of heredity, based as they are on a splendid array of facts, will be of especial influence upon future theories of heredity and development.

Together with phylogeny the science of palæontology, founded by Cuvier, developed mainly through the influence and personal activity of such men as Joachim Barrande (1799-1883), Jean-Baptiste-Julien d'Omalius d'Halloy (1783-1875), James Dwight Dana (1813-95), Oswald Heer (1809-83), and many more. These giants in the natural sciences were at the same time faithful Christians, the first two being Catholics. Still more impressive than the progress of palæontology is that of systematic biology and bionomics, branches to which a thousand modern scientists have devoted the entire energy of their lives. The result of all this scientific activity is apparent in the immense collections preserved in the museums of Washington, London, New York, and other large cities, and in the simple fact that the systematic species scientifically described amount to no fewer than 500,000 animals and 200,000 plants. The Linnæan system of classification was perfected in many ways, especially by the botanists A.L. von Jussieu (1789), A. P. Decandelle (1813), and by the zoologists Cuvier, C. T. E. von Siebold (1848), and R. Leuckart (1847). The greatest of modern morphologists since the time of Albrecht von Haller are Richard Owen (1870-92), the comparative anatomist, Johann Müller, the father of German medicine, and Claude Bernard, the prince of physiologists. Müller was born 14 July, 1801, at Coblenz, and died 28 April, 1858, as professor of anatomy and physiology in the University of Berlin. He was the teacher of such well-known men as Virchow, Emil Dubois-Reymond, Helmholtz, Schwann, Lieberkühn, M. Schultze, Remak, Reichert, all of whom have done magnificent work in various departments of biology. Müller was chiefly an experimental physiologist, and established a vast number of facts which he described with great accuracy. At the same time he defended with energy the existence of a special vital force, which directs the various physical and chemical forces for the attainment of specific structures and functions. In the present generation biologists are gradually returning to Müller's views, which for a time they had more or less completely abandoned. The great physiologist lived all his life, as he died, a faithful Catholic. The same may almost be said of his contemporary in France, Claude Bernard born in 1813, at St.-Julien, not far from Lyons, and died in 1880. Bernard's main discoveries refer to the phenomena of nervous inhibition and internal glandular secretion. For a time he yielded to the materialistic philosophy of his age, but he soon abandoned it, perhaps through the influence of his friend Pasteur.

Louis Pasteur (died 28 September, 1895), the father of preventive medicine, was probably the most gifted and influential biologist of the nineteenth century. His discoveries, which are inscribed on his tomb, in the Institut Pasteur, at Paris, extend from 1848 to 1885, and relate to the nature of fermentations, to the minutest organisms and the question of abiogenesis, to the diseases of silkworms, to the propagation of diseases by microbes, and above all to the supremely important principle of experimental immunity to pathogenic bacteria. Pasteur was a model Catholic, the most ideal scientist known in the history of biology.

Many more prominent biologists, such as Ramon y Cajál, Wundt, Brooks, Strassburger, Wasmann, have done and are still doing admirable work in the interest of biological sciences.

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